The genus Cypris belongs to this group. Several species may be seen swimming in our streams and fresh-water pools. The body of the common Cypris (fig. 151) is enclosed between two flat oval shells, united by a hinge on the back. The little animal can open and shut the valves by means of two slender muscles, extending from its back to the shells, which are much curved above and rather flat below. There are two pairs of antennæ beneath the eye, they are perfectly transparent, many-jointed, and end in tufts of filaments. One pair projects forward and then bends gracefully backwards; the other pair are bent downwards. The mouth has no foot-jaws, and there are only two pairs of feet. Only one pair is seen in the female, for the other pair is bent upwards to support the egg sacs. The Cypris attaches her eggs to the leaves of aquatic plants by a greenish fibre. Not more than twenty or thirty eggs are deposited by one individual, while the heaps contain several hundreds; so many females contribute to form one heap. The young are hatched in the form of their parent in about four days and a half. As the pools dry up, the Cyprides bury themselves in the sand or mud at the bottom; if that remain moist they survive, if it becomes dry they perish; but the eggs remain dormant till the return of rain, when they are hatched, and the surface of the water is soon crowded with a swarm of young Cyprides. [Illustration: Fig. 152. Section of Daphnia pulex.] The Cladocera is the first order of the gill-footed Crustacea: their body is defended by a bivalve carapace; they have from four to six gill-footed limbs, one compound eye, and two pairs of antennæ, one pair of which is large and adapted for swimming. The Daphnia pulex, or Arborescent Water-flea, of which fig. 152 is a section, is a common form of this tribe. It is very abundant in pools and ditches, coming in groups to the surface in the mornings and evenings in cloudy weather. The bivalve shell is transparent, flexible, and open below; it ends behind in sharp toothed peaks. The eye placed in front is moved by four muscles, and on each side of it are the great antennæ, which are jointed, branched, and garnished with feathery filaments, and are the chief organs of locomotion. This animal has no foot-jaws, but it has a nervous system and a heart, whose pulsations are repeated two or three hundred times in a minute, and the blood is aërated by gills at the extremities of six pairs of bristly feet situated behind the mouth, and only used for respiration and prehension.